1. Field of the Invention
The present invention relates to an aircraft electric generation system adapted to be connected with an aircraft engine for driving an electric generator at a constant number of revolutions.
2. Background Art
Conventionally, the so-called IDG (Integrated Drive Generator)-type system has been known as a large-sized aircraft electric generation system. In general, the electric generation system of this type uses a continuously variable transmission (CVT) and is adapted for rotating the electric generator at the constant number of revolutions, irrespective of the number of revolutions of the engine (or engine speed), thus allowing the electric generator to generate electric power of a constant frequency. Further, as the IDG-type electric generation system, one system, which is composed of a continuously variable transmission (CVT) combined with a planetary gear transmission that can serve to compensate for relatively low mechanical efficiency of the continuously variable transmission (CVT), has been known. This system includes the so-called “Power-Split-Type Constant Speed Drive Unit,” which is configured to split and share the driving power to be transmitted between the two transmissions, by using a power split shaft (see Patent Document 1).
By the way, as the aircraft engine, a two-shaft-type engine including a hollow high-pressure shaft connecting a compressor with a high-pressure turbine and a low-pressure shaft extending through the high-pressure shaft and connecting a fan with a low-pressure turbine has been well known. In the aircraft electric generation system including the power-split-type constant speed drive unit, the planetary transmission is operated at a fixed transmission gear ratio so that the transmission gear ratio of the entire constant speed drive unit is limited within a range of about 2:1 or less. Therefore, in the case of the aforementioned two-shaft-type engine, an input shaft of the constant speed drive unit is connected with the high-pressure shaft with less variation in the number of revolutions. Normally, the aircraft electric generation system is installed, together with a gear box, below the aircraft engine (see Non-patent Document 1).
Patent Document:
Patent Document 1: JP3440287B
Non-patent Document:
Non-patent Document 1: Richard C. McClung, “Constant Speed 400 Hz Aircraft Electric Generation System,” Aerospace Meeting, Hyatt House (Airport), Los Angeles, Dec. 3-6, 1979
In general, the output of the conventional IDG-type electric generation system has been approximately 90 kVA. However, from recent marked increase of the number of various electric and electronic parts or units introduced into each aircraft body, it can be expected that the capacity of electric generation of such an aircraft is required to be greater than 200 kVA. However, in order to increase the electric generation capacity up to such a level (i.e., 200 kVA or higher), it is necessary to increase so much the size of each disk or the like of the continuously variable transmission (CVT), thus enlarging the size of the entire electric generation system. In particular, in the power-split-type electric generation system, the input shaft of the continuously variable transmission (CVT), a rotation shaft of the electric generator and the power split shaft are respectively arranged in parallel with one another, while constituting together a triangle, when seen in the axial direction of the shafts. Therefore, such arrangement of the shafts should occupy a considerably large space, causing the electric generation system itself to rather project or protrude downward from the aircraft engine. Of course, such arrangement of these shafts should increase the area of a front face of a nacelle covering both of the engine and electric generation system, thus also increasing the drag against the aircraft body so much. This is rather disadvantageous to the fuel consumption and emission efficiency.
Besides, in the case in which the electric generation system is connected with the high-pressure shaft of the aircraft engine and if the total electric load of the aircraft is increased greater than 200 kVA, there is a risk that some serious engine operability problem would occur. However, if the electric generation system is connected with the low-pressure shaft that has no limitation on power extraction, it would be quite difficult to control the electric generator to be rotated, at a constant speed, within the transmission gear ratio of 2:1 or less, because of rather great variation in the number of revolutions of the low-pressure shaft.